As an example of a conventional signal processing system for a sensor for judging an event to be detected on the basis of a frequency of a sensor output, a piezoelectric sensor system, which detects force generated by shock or oscillation and outputs voltage signal, is known. The piezoelectric sensor system can be employed for a pinch-detection apparatus for an electric opening/closing apparatus which opens/closes a slide door with a motor or the like, such as an automatic door of a building, an electric slide door for a vehicle such as a wagon and a van.
Piezoelectric element generates electric polarization in accordance with external force (stress). The piezoelectric sensor utilizing the piezoelectric element outputs an oscillating voltage signal such as a sinusoidal wave generated by electric polarization in accordance with external force applied when an object is pinched by the opening/closing door or the like. Not only a pinch, but also an vibration or the like, induces voltage output. However, because of difference in frequencies of the voltage waveforms, an event to be detected (in this case, pinch of an object) can preferably be identified. Further, because the piezoelectric sensor starts generating voltage at an early stage of pinch, in which external force is weak, pinch can be detected at the beginning of the event. Accordingly, a countermeasure can be conducted against the pinch before large force is applied to the pinched object JP2003-106048A. describes a pressure sensitive sensor utilizing such a piezoelectric sensor applied to a hatchback door for a vehicle.
However, as described in JP2003-106048A, the frequency of the voltage output transmitted when the pinch is detected is extremely low, approximately 10 Hz or less. Accordingly, measuring the waveform during one cycle of a time period for determining the frequency takes inverse number of frequency, in other words, 100 milisecond or more. In the pressure sensitive sensor described above, advantages of a highly sensitive piezoelectric sensor, which can conduct detection of early stage of pinch, are not satisfactorily utilized
A need thus exists for a signal processing system for a sensor for analyzing a sensor output in a short time and for judging whether an event to be detected has occurred on the basis of a frequency of the sensor output.